Atrial fibrillation (AF) is the most common diagnosed arrhythmia, with the number Americans diagnosed with AF estimated at 2.2 million and continuing to increase as the population ages. Electrical activity during AF is characterized by complex activation patterns that are difficult to map. However, there is growing evidence that AF is driven by stable sources in the atria, such as rotors or focal activity. The most widely used ablation and surgical interventions to treat patients with AF use generalized empiric strategies that target that do not consider the specific mechanisms of the arrhythmia. To date, there are no established methods utilizing AF electrograms that effectively locate AF sources in the atria. We have recently developed an electrogram morphology recurrence analysis that has been able to identify areas of the atria that have high rates of electrogram morphology recurrence. Our preliminary study has shown that in patients who underwent ablation targeting the pulmonary veins, when the electrograms with highest recurrence rates were located in the left (versus right) atrium ablation success rates were higher. In contrast, having these fast/recurring electrograms in the right atrium indicated a very poor ablation success rate. Additionally, high resolution electrical mapping performed in a canine model of AF showed that morphology recurrence mapping could identify areas of rotor activity. These preliminary data suggest that a mapping and ablation strategy based on morphology recurrence analysis could be used in the treatment of AF. Phase I of this study involves the development of a prototype mapping software that can generate electrogram morphology recurrence maps in a real-time clinical setting and the evaluation of this system on ablation in ten patients who have previously failed conventional AF ablation. The mapping software will collect digital recordings from a basket catheter positioned in the right atrium and then the left atrium. The maps will provide easy identification of the site with high morphology recurrence, which can be related back to the anatomy with electroanatomic mapping. If sites of high morphology recurrence rates represent areas at or near rotors or focal sources, then we expect either acute termination of the AF or significant slowing of the AF activation rate. A successful Phase I study -showing that ablation via recurrence mapping produces AF termination or slowing in the majority of the ten patients - would support the need for a larger study with longer term follow-up. Commercialization possibilities for this mapping approach include the production of a stand-alone system or integration into an existing mapping system.